Our invention is adapted to be used in a fuel system for a so-called flexible fuel vehicle, also known as an FFV. Such vehicles have been described in automotive technical literature as a possible solution to environmental concerns about the use of petroleum fuels and to marketing concerns arising from over-dependence on petroleum fuels as a source of fuel for internal combustion engines.
Methanol is considered to be an acceptable additive for gasoline fuels in the formulation of a commercially feasible gasoline/alcohol mixture for a flexible-fuel vehicle. The combustion characteristics of methanol, however, are substantially different than the combustion characteristics of gasoline. Gasoline fuels that presently are available commercially, therefore, cannot be used interchangeably with a mixture of methanol and gasoline without appropriate adjustments of the air/fuel ratio and the timing of the spark advance for spark-ignition internal combustion engines. The energy content of methanol, for example, is about half of the energy content of gasoline per unit volume. Methanol also tends to increase the cold start problems during engine cranking, especially in cold weather.
Engine controls for flexible-fuel vehicles require a design compromise between fuel performance, fuel cost and effects of methanol content on engine fuel and air mixture control and on ignition timing. A design objective in designing controls for flexible fuel vehicles is the achievement of high engine performance and stoichiometric fuel combustion efficiency with varying methanol concentrations, even with gasoline and methanol mixtures of up to 85 percent methanol.
Electronic powertrain controllers of the kind commonly used in the automotive industry are capable of adjusting engine ignition timing, fuel rate and air-fuel ratio to satisfy a variety of operating variables including vehicle speed, altitude, ambient temperature, automatic transmission ratio and throttle setting. The flexible fuel sensor of our invention is capable of providing additional information to such a powertrain controller so that the controller may compensate for variations in the concentration of methanol in the gasoline fuel as well as for other driveline variables.
A characteristic of alcohol that makes it feasible to obtain a reliable measure of alcohol concentration is its relatively high dielectric constant in comparison to the dielectric constant for gasoline. In the case of pure methanol, for example, the dielectric constant may be approximately 32, whereas in the case of gasoline the dielectric constant may be approximately 2.0. The dielectric constant for pure ethanol is about 25. These relatively wide variations make possible useful measurements by the sensor that can be reliably processed by the powertrain controller.
We are aware of prior art teachings that rely upon the use of a capacitor in the presence of a methanol gasoline fuel mixture to provide variations in the dielectric constant for the capacitor as an indication of the percentage of methanol in the mixture. One such teaching is shown in prior art U.S. Pat. Nos. 4,971,015 and 4,915,084. These patents describe a sensor that comprises a capacitor situated in a fuel line of an internal combustion engine. One capacitor element of the sensor is located in the fuel flow path, and a companion capacitor element surrounds it and defines a part of the fuel delivery conduit. An oscillator circuit, which is independent of the sensor, develops an oscillating voltage that drives an oscillator comprised of the capacitor and a series related resistor. The resonant frequency of the circuit changes in response to changes in the capacitance of the sensor due to changes in the dielectric constant of the fuel mixture, the latter in turn being a measure of the percentage of methanol in the fuel.
It is also known in the art to provide a so-called optical sensor for measuring alcohol concentrations in gasoline, an example being described in U.S. Pat. No. 4,438,749. Such sensors rely upon a light source and include an electronic circuit that measures the light transmission through the fuel mixture as the refractive index is adjusted depending upon the concentration of the alcohol in the fuel. Such systems are relatively undependable, however, because of variations in the light transmissibility of the fuel due to changes in the mixture that do not demand a corresponding change in air/fuel ratio.
Other prior art sensors that rely upon an oscillator circuit to detect variations in fuel mixture comprise a microwave oscillator of the kind described in U.S. Pat. No. 4,453,125, in which a capacitor circuit has a frequency that is affected by changes in the dielectric constant of the fuel blend. The attenuation of the microwave frequency is an indicator of the fuel additive content. Such systems, however, are relatively expensive and generate a relatively high electromagnetic interference level.
Another optical sensor is described in U.S. Pat. No. 4,995,367. The fuel control system of the '367 patent controls droplet size of fuel particles in a methanol/gasoline engine to effect efficient combustion. The system provides for adjustments in the air/fuel ratio and in ignition timing. One of the variables used by the controller is the methanol percentage for the fuel. The fuel sensor used to determine that percentage is an optical unit with a photoelectric pickup.
Another prior art methanol sensor is described in technical literature of Japan Electronic Control Systems Co., Inc. of Isesaki, Japan. This sensor, like the sensor described in the '084 and '015 patents mentioned above, uses concentric capacitor electrodes through which fuel is passed. A separate oscillator circuit develops a frequency that is modified by the fuel capacitor as the methanol content changes. An alcohol concentration sensor forming a part of an electronic engine fuel control is shown in U.S. Pat. No. 5,003,956, which is assigned to Japan Electronic Control Systems Co., Inc.